A mask is also referred to as a photo-mask. Currently, a phase-shift mask is available. The phase-shift mask comprises a substrate, on which a light transmission region and a light shading region are formed while a phase-shift region and a non-phase-shift region are formed. During an exposure process using the phase-shift mask, the phase of the light transmitted through the phase-shift region is changed by 180°, and the phase of the light transmitted through the non-phase-shift remains unchanged, thereby destructive interference occurs between the light transmitted through the phase-shift region and the light transmitted through the non-phase-shift region, and an objective to better control the critical dimension (CD) of a pattern to be formed can be achieved.
FIG. 1A shows a structure of a phase-shift mask usually used for forming a source, a drain and a data line of a thin film transistor (TFT) in the prior art. FIG. 1B shows a sectional view of the phase-shift mask in FIG. 1A along the line A-A. As shown in FIGS. 1A and 1B, the phase-shift mask comprises a substrate 1 and a light shading pattern 3 formed on the substrate 1. The substrate 1 has a relatively stable thermophotovoltaic property, and is usually formed of transparent medium such as quartz or the like. On the substrate 1, a region with no light shading pattern 3 formed thereon is referred to as a light transmission region, and the light shading pattern 3 corresponds to a light shading region. The light shading pattern 3, for example, is formed with a light shading and phase-shift film or comprises a light shading layer and a phase-shift layer, that is, the light shading region corresponds to the phase-shift region and the light transmission region corresponds to the non-phase-shift region. For example, a phase-shift layer with a low light transmittance (about 8%), as the light shading pattern 3, may be formed on the substrate 1 by performing deposition, photolithography, development, etching and stripping of pattern material. The phase-shift layer is usually formed of a material with a low light transmittance and capable of reversing the phase of light (i.e., shifting the phase of light by 180°±10°). When the phase-shift mask is used to form a source, a drain and a data line of a thin film transistor, the phase of the light transmitted through the phase-shift region of the phase-shift mask is reversed (i.e., the phase of the light is shifted by 180°), and the phase of the light transmitted through the non-phase-shift region of the phase-shift mask remains unchanged, thus destructive interference occurs between the light transmitted through the phase-shift region and the light transmitted through the non-phase-shift region, thereby the resolution of the pattern formed by using the phase-shift mask is relatively high.
One development trend of the panel display technology is to obtain a high resolution. To achieve this objective, the phase-shift mask is regarded as an important technology to increase the resolution and is thus used for manufacturing a thin film transistor. However, from simulation and actual measurement, it can be found that, as an additional pattern is connected to a symmetrical pattern to form the light shading pattern of the phase-shift mask shown in FIG. 1A, during an exposure process using the phase-shift mask, the exposure intensity distribution corresponding to the symmetrical pattern may be asymmetrical, resulting in that a pattern which should be formed to be symmetrical is actually asymmetrical. As shown in FIGS. 1A and 1B, the U-shaped pattern of the phase-shift mask is used for forming a source of a thin film transistor and is referred to as a source pattern 31, and the stick-shaped pattern inserted into the recessed part of the U-shaped pattern is used for forming a drain of the thin film transistor and is referred to as a drain pattern 32. The U-shaped pattern and the stick-shaped pattern constitutes the symmetrical pattern, and an additional pattern used for forming a data line of the thin film transistor is connected to the right side of the U-shaped pattern, and the additional pattern is referred to as a data line pattern 33 and is connected with the source pattern 31. Due to the connection of the additional pattern, during an exposure process using the phase-shift mask, the exposure intensity distribution corresponding to the symmetrical pattern (in particular, the source pattern 31) may be influenced to be asymmetrical. FIG. 1C shows a diagram of the exposure intensity distribution corresponding to the symmetrical pattern during an exposure process using the phase-shift mask shown in FIG. 1A. As indicated by the dotted line in the FIG. 1C, the light transmittance intensity at the side of the source pattern 31 connected with the data line pattern 33 is relatively weak, and the light transmittance intensity at the other side of the source pattern 31 with no data line pattern 33 connected thereto is relatively strong. Therefore, when using the above phase-shift mask to form a source, a drain and a data line of a thin film transistor, the thin film transistor may have poor performance due to the asymmetry of the formed source. For example, the ratio between the width and the length of the channel of the TFT (i.e., the width of the channel/the length of the channel, W/L) may be changed so that the turned-on current Ion of the TFT is reduced, thereby the display quality of a display panel comprising the thin film transistor may be affected.